The present invention relates to a color image forming apparatus which can copy images of a plurality of colors, for example an electrostatic color copier.
An electrostatic copier which can copy images of red, blue and black, the maximum number of colors being three, is widely known.
In this type of color copier, an image of three colors can be copied as follows: a photoreceptor drum is rotated three times, and the primary scanning is conducted three times by an image reading section so that three color images can be formed in the image region on the drum surface, wherein an image of one color is registered on an image of another color. In the way described above, one copying operation is completed when the photoreceptor drum has been rotated three times, and primary scanning has been conducted three times. Specifically, a red image is formed in an image region on the photoreceptor drum surface by the first primary scanning (a scanning for red) which is conducted during the first rotation of the photoreceptor drum; a blue image is formed and registered on the red image formed previously in the image region by the second primary scanning (a scanning for blue) which is conducted during the second rotation of the photoreceptor drum; and a black image is formed and registered on the image formed previously in the image region by the third primary scanning (a scanning for black) which is conducted during the third rotation of the photoreceptor drum. The registered images of three colors are transferred onto a sheet of transfer paper. In the same manner described above, when an image of two colors is copied, one copying operation is completed when the photoreceptor drum has been rotated twice, and primary scanning has been conducted twice. When a monocolor image is copied, one copying operation is completed when the photoreceptor drum has been rotated once, and primary scanning has been conducted once.
In this kind of copier, a copier provided with an automatic color selection mode is widely known, in which pre-scanning is conducted before the primary scanning in order to judge the conditions of document colors so that the color to be used and the copy process to be conducted can be automatically determined.
When a copy operation according to the automatic color selection mode is conducted by this copier, in the case of a document of three colors, primary scanning is conducted three times after one pre-scanning. Namely, one copy operation is completed when the document image has been scanned four times. In the case of a document of two colors, primary scanning is conducted twice after one pre-scanning so that one copy operation is completed when a document image has been scanned three times. In the case of a monocolor document, one primary scanning is conducted after one pre-scanning so that one copy operation is completed when a document image has been scanned twice.
As explained above, in the case of a conventional color image forming apparatus performing the automatic color selection mode, pre-scanning is necessarily conducted once before the primary scanning. Accordingly, the time necessary for the prescanning is needed in addition to the time necessary for the primary scanning.